The present invention pertains to the preparation of polyester resin materials via the reaction of a polybasic acid or anhydride with a polyol reactant. More particularly, the present invention relates to an improved process for preparing polyester resins of the sort wherein the polyol reactant contains at least a minor proportion of an alkyl, alkenyl or aryl glycoside component.
Polyester resins are a known class of polymeric compounds which are generally thought of or characterized as being the reaction product of polyhydric alcohols and polybasic acids. Such resins can vary widely in terms of their physical and chemical characteristics or properties depending upon the identity and nature of the specific polybasic acids and polyols chosen for their manufacture, the molecular weight of the final polyester product, and the like. Such resins find a wide range of practical uses such as in molded articles, paint and coating applications, adhesive applications, etc.
Alkyd resins are a particular type of polyester resin, namely polyester resins which have been modified with monobasic fatty acids, which find widespread and beneficial use in protective coating applications such as, for example, in industrial coatings, paints and the like. See for example U.S. Pat. No. 4,181,638 to Lasher (issued Jan. 1, 1980) which discloses a crosslinkable, high solids polyester resin-based coating composition. Said composition comprises a low molecular weight polyester resin and an aminoplast resin which provides cross-linking upon cure. According to this patent, the low molecular weight polyester resin component is preferably prepared by reacting a cyclic dicarboxylic acid or anhydride with a mixture of mono- and di-fatty esters of triols such as glycerol, trimethylol propane and the like. In preparing said polyester resins, the reactants are charged to a heated reactor vessel along with a reflux solvent such as n-heptane, xylene or toluene and an acid catalyst such as phosphoric acid or sulfuric acid; the reaction mixture is blanketed with an inert gaseous atmosphere (e.g., carbon dioxide); and the reaction is conducted at a temperature below about 150.degree. C. until the reaction is about 90 percent complete, after which the reaction is completed at a temperature of from 175.degree. to about 190.degree. C.
Any number of polybasic acids and polyhydric alcohols can theoretically be employed to prepare polyester resins. However, as a practical matter, phthalic anhydride is one of the more commonly used polybasic acids, particularly for alkyd resin applications, and glycerol and pentaerythritol are quite commonly used as the polyhydric alcohol component. Frequently, mixtures of pentaerythritol with glycerol or with ethylene glycol are used to effectively reduce the pentaerythritol functionality and to thereby control the degree of crosslinking in the resulting pentaerythritol-based polyester resin product. In alkyd resin systems, mixtures of phthalic anhydride with a minor proportion (e.g., from 1 to 10 weight percent based on phthalic anhydride weight) of maleic or fumaric acid are sometimes employed to achieve improved color, processing time, and water resistance in phthalic anhydride-based alkyd resin products.
As is noted briefly above, it is common practice to employ selected mixtures of polybasic acids and/or selected mixtures of polyhydric alcohols to control the ultimate physical and chemical characteristics of the polyester resin material to be produced. For example, one of the various polyhydric co-polyols which has been suggested for use in alkyd resin manufacture is methyl glucoside. For example, in an article by J. P. Gibbons entitled "Methyl Glucoside," Paint and Varnish Production, Vol. 48, No. 11, October 1958 (pages 57-63) there is discussed the preparation of methyl glucoside-containing oil-modified alkyd resins by alcoholysis of triglycerides with methyl glucoside followed by reaction of the resulting alcoholysis product with dibasic acid materials such as tetrahydrophthalic or maleic anhydrides, or fumaric, succinic, adipic, sebacic or azelaic acids. In said article, it is noted that when methyl glucoside-containing oil-modified alkyd resins of the sort described above are prepared using phthalic anhydride as the dibasic acid component very dark-colored alkyd resin products are produced.
Since alkyl glycosides such as methyl glucoside represent economically attractive polyol components for potential use in polyester resin manufacturing operations, it would be highly desirable to provide a process by which alkyl (or alkenyl or aryl) glycoside-containing polyester resins having improved color characteristics could be satisfactorily manufactured. Additionally, since phthalic anhydride is a commonly used dibasic acid component in polyester resin manufacture, it would be extremely desirable to provide a process by which alkyl (or alkenyl or ary) glycoside-containing, phthalic anhydride-based polyester resins having commercially acceptable color characteristics could be satisfactorily produced.